The present invention refers to a method of combustion in a combustion chamber enclosing a pressurized fluidized bed and a space located above the bed, comprising the steps of: feeding an oxygen-containing gas into the bed; supplying a fuel to the bed; supplying a particulate absorbent to the bed for the absorption of undesirable substances released during the combustion; collecting in the space combustion gases formed during the combustion; discharging the combustion gases from the combustion chamber; and separating solid material from the combustion gases. Moreover, the invention refers to a combustion plant comprising a combustion chamber, which is provided to enclose a pressurized fluidized bed and a space located above the bed and in which a combustion of a fuel is intended to be performed while forming combustion gases, at least a first fuel supply member arranged to supply the fuel to the bed, means arranged to supply an oxygen-containing gas to the bed, means arranged to supply an absorbent to the combustion chamber, and a purification device for purifying the combustion gases, the purification device comprising a separating member arranged to separate particulate material from the combustion gases.
It is known to combust different fuels in a bed of particulate, incombustible material, in which the bed is supplied with combustion air from beneath through nozzles in such a manner that the bed becomes fluidized. One differs between different types of such combustion in a fluidized bed, which operate according to different principles and under different conditions. Firstly, one differs between an atmospheric bed and a pressurized bed. In comparison with an atmospheric bed a pressurized, fluidized bed is characterized by a small plant size in relation to the effect produced, by a high efficiency, and in that the combustion occurs under advantageous conditions from an environmental and economical point of view. A pressurized bed may have a larger height than an atmospheric bed since one may operate with greater pressure drops. Among the atmospheric beds, circulating beds are frequently used in which the bed material is permitted to circulate through a separating device in order to be recirculated to the bed. In such a way possibly unburnt fuel may be recirculated, which improves the efficiency of the combustion, and also absorbent material not used for absorption of in the first place sulphur, which decreases the discharge of contaminants from the combustion. However, such circulating beds operate with relatively high fluidizing velocities, in typical cases in the order of 5through 12 m/s. Fluidizing velocity means the velocity that the gas would have had if it would have flowed through the combustion chamber without the presence of particles. This causes problems with erosion on, for instance, the steam tube arrangement provided in bed in such a way that the lifetime thereof significantly decreases. Furthermore, one may discern the so called bubbling beds in which the fluidizing velocity is relatively low, in typical cases between 0.5 and 2 m/s. Such a bed is relatively well defined in a vertical direction and there is formed a space, a so called freeboard, in the combustion chamber above the bed. In this freeboard a relatively small amount of dust particles are present in comparison with a circulating bed but there is essentially no pressure drop across the freeboard.
Recently, attempts have been made to provide a certain circulation also in pressurized beds by supplying the combustion gases leaving the combustion chamber to a cyclone for separation of solid material, which is recirculated to the combustion chamber. In order to obtain the desired effect concerning the degree of utilization of the absorbent and the combustion efficiency by the recirculation, the solid material should be supplied at the bottom of the fluidized bed. This means that the pressure drop which is present in the bed and in the cyclone, in typical cases about 0.5 bars, has to be overcome.
In order to overcome this pressure drop, it has been suggested to provide a dosing device, for example of a cell feeding type, at the end of a recirculating pipe provided preferably vertically and connecting the cyclone to a combustion chamber. The dosing device may comprise a rotatable shutter provided on the pipe and having a weight which in normal cases keeps the shutter in a closed position. When the amount of material in the pipe is sufficient, the weight thereof will overcome the weight of the shutter which means that the shutter is opened and the material is discharged. Such a device leads to an intermittent recirculation of solid material. However, such devices do not function in the way intended in the environment of a fluidized bed due to the movements occuring in the bed and the forces caused by these movements. Furthermore, such devices are rapidly destroyed due to the aggressive, erosive and corrosive environment.
Another solution is an L-valve located in the bed and having a vertical portion in which a column of material is built up. In order to provide a flow of material through the channel, such a device requires that gas is injected in the lower portion of the L-valve and, in order to provide stability, it is necessary to continuously measure the height of the column of material, which is very difficult, if not impossible, in the actual environment.
SE-B-460 148 suggests another way of overcoming this pressure drop. SE-B-460 148 discloses a combustion plant having a combustion chamber enclosing a pressurized fluidized bed for the combustion of a fuel while forming combustion gases. Furthermore, the plant comprises a purification of the combustion gases in several stages. In a first stage, particulate material is separated by means of a cyclone from the combustion gases and supplied to a collection chamber beneath the cyclone. Via a horizontal recirculating channel, the collected dust particles are fed back to the combustion chamber in order to improve the use of unburnt fuel and absorbent material. The recirculation is accomplished by means of an air-driven ejector blowing the material into the combustion chamber. However, such an air injection is very expensive. The gain of the absorbent utilization and the combustion efficiency is lost in the effect for the compressor providing primary air to the ejector. In addition this method leads to erosion.
It should be noted that the recirculation of solid material separated from the combustion gases means that the recirculated fine part may provide as much as 10-40% of the mass of the bed, which strongly influences the heat transfer coefficient to the tubes located in the bed. The fine part is comprised of particles having a largest diameter of about 300 to about 400 .mu.m and an average particle diameter of about 50 to about 150 .mu.m.
U.S. Pat. No. 4,021,184 discloses a combustion plant developed for the combustion of waste material. The plant comprises a combustion chamber for a recirculating fluidized bed. The bed disclosed in this reference is not pressurized but the plant operates at atmospheric pressure and is of a diluted type (dilute phase fluidized bed), i.e. the fluidized bed fills up the whole combustion chamber. Such a type of bed means that a very large part of the solid, hot bed material will be transported out from the combustion chamber together with the combustion gases formed during the combustion. Therefore, it is suggested that cyclones for separating dust particles from these gases be provided at the outlet of the combustion chamber and that the separated, hot dust particles are recirculated to the combustion chamber via conduit pipes connecting the cyclones with the combustion chamber. In such a manner it is possible to recover the heat energy in the dust particles leaving the combustion chamber. Thus, a recirculation may be obtained due to the low pressure drop across the bed, i.e. the whole combustion chamber. In addition, the valve mentioned (trickle valve) in the end of the conduit pipe is probably necessary.
EP-B-176 293 discloses another combustion plant having a combustion chamber which encloses a fluidized bed and in which combustion of a fuel is intended to be performed while forming combustion gases. The bed is of a bubbling type but the combustion chamber operates at atmospheric pressure. Furthermore, the plant comprises a cyclone for separating particulate material from the combustion gases and provided above the combustion chamber. The particulate material separated is conducted via a pipe back into the bed by letting the material simply fall freely through the pipe. This is possible since the bed disclosed in this document has a relatively low height, about 1 m. In addition the pressure drop is relatively small.
U.S. Pat. No. 4,103,646 discloses a plant comprising two combustion chambers, the first having a fast fluidized bed, i.e. the fluidizing velocity is between 7 and the 10 m/s, and second having a "slow", bubbling fluidized bed. The combustion gases formed in the first combustion chamber are conducted to a cyclone, where solid material is separated and fed to the second combustion chamber. In the bottom of the second combustion chamber there is a discharge channel for solid material which is then recirculated to the first fast combustion chamber by means of air injection.
SE-B-470 222 discloses a method of combustion in a combustion chamber enclosing a pressurized fluidized bed and a space located above the bed. In addition, in the combustion chamber there is a tube arrangement for generating steam to a steam turbine. The combustion is performed by feeding oxygen-containing gas into the bed and by supplying a particulate fuel to the bed. The combustion gases generated during the combustion are collected in the space and are then conducted away from the combustion chamber. Furthermore, it is known from SE-B-470 222 to raise the temperature of the combustion gases by the combustion of a complementary fuel injected into the freeboard. This combustion is especially utilized during part load operation of the plant in order to adapt the temperature of the combustion gases to an optimal temperature for the subsequent gas turbine. Such a freeboard combustion functions appropriately by the addition of such complementary fuels as volatile oils or gases. However, it is disadvantageous to need several different types of fuels for a single plant, since this makes the handling and the operation of the plant more complicated. Certainly, SE-B-470 222 suggests using fine grounded carbon as fuel for the complementary combustion. Such a combustion of carbon is disadvantageous since the combustion gases from the complementary combustion will not pass the bed, in which there is a sulphur absorbent, prior to leaving the plant. Thus, the sulphur dioxide formed during the combustion in the freeboard will pass directly to the atmosphere and will not be bound by a sulphur absorbent. Another problem of this method is that it may be difficult to combust a fuel, such as carbon, in the freeboard during part load since the temperature in the freeboard then may be too low.